Methane synthesis from CO2 and H2O using electrochemical cells with polymer electrolyte membranes and Ru catalysts at around 120 °C: a comparative study to a phosphate-based electrolyte cell†

Abstract

Electrochemical cells with fluorine-based polymer electrolyte membranes (PEMs) and Ru catalysts have been investigated for the production of methane (CH₄) from CO₂ and H₂O by using electricity at 120 °C. CH₄ was synthesized with a rate of 12 nmol s⁻¹ cm⁻² at a current density of 10 mA cm⁻² with a CO₂ flow of 0.055 ml_STP min⁻¹ in the cathode vessel and with an Ar + H₂O flow of 10 ml_STP min⁻¹ + 10 μL_liquid min⁻¹ in the anode vessel (STP; standard temperature and pressure at 0 °C and 101.3 kPa, respectively). This rate was corresponding to the current efficiency of ca. 85%, and unreacted H₂ and subproducts of CO were obtained with current efficiencies of ca. 14 and 1%, respectively. The properties of temperature and current density dependence were discussed in this article. The present system exhibits significantly higher selectivity in synthesizing CH₄ compared to electrochemical CO₂ reduction systems at the electrode/electrolyte interfaces. Electrochemical CO₂ reduction did not take place in the present system, and the importance of the combination of water electrolysis and catalytic methanation at solid/gas interfaces was proposed. A comparative study between phosphate (CsH₂PO₄/SiP₂O₇) at 250 °C and polymer electrolytes at 120 °C was performed and the possibility for practical applications of both systems was discussed.